1. Field of the Invention
This invention relates to a spark plug having a center electrode and an outer electrode, at least one of which is made of a nickel-alloyed clad and a thermally conductive copper-alloyed core embedded in the nickel-alloyed clad.
2. Description of Prior Art
In a spark plug for use in internal combustion engine, a center electrode is made of a nickel clad and a copper core embedded in the nickel clad. When the engine runs repeatedly between full throttle and idle operation, the composite electrode is exposed to a huge temperature differential environment so that the nickel clad plastically deforms due to the thermal stress caused from the thermal expansional difference between the nickel clad and the copper core. The increased thermal stress causes to unfavorably deform the center electrode. The degree of the deformation depends upon the growth of void developed in the copper core. The relationship with the void is such that the fully grown void accelerates the deformation of the nickel clad of the center electrode.
FIG. 11a shows how the center electrode 110 deforms depending upon the void 130 grown in the copper core 120c embedded in the nickel clad 120n due to the repeated thermal stress. The grown void 130 causes to radially expand and axially contract the center electrode 110 from the phantom line position to the solid line position.
When the engine alternately runs 6000 cycles between 5000 rpm full throttle for one minute and idling operation for one minute, the center electrode 110 further undergoes the repeated thermal stress to continue expanding radially so as to finally develop cracks 140c in an insulator 140 as shown in FIG. 11b.
Meanwhile, when the composite structure of nickel clad 160n and copper core 160c is applied to an outer electrode 150, voids 170 grows in a copper core 160c due to the thermal expansional difference between the nickel clad 160n and the copper core 160c. As shown by the phantom line in FIG. 12, the fully grown voids deform the outer electrode 150 away from a front end 151a of a center electrode 151.
As understood from the above description, the deformation of the two electrodes 110, 150 is due to the voids 130, 170 grown in the copper core 120c, 160c. It is, therefore, necessary to control the growth of these voids to prevent the deformation of the electrodes.
For this reason, various types of copper-based alloy has been investigated, and a number of patent applications have been filed and Patent Provisional Publication Nos. 61-143971, 61-143972, 61-143973, 61-148788, 61-148789, 61-148790 and 4-065791.
Among these patent applications, the laying-open patent application No. 61-143973 discloses a copper-alloyed core containing an element or elements in the range of 0.03-1.0 weight percentages selected from the group consisting of Ti, Zr and Cr.
All these patent applications are intended to select specific elements to add them to the copper core in a certain percentage range, and none of the patent applications discloses how the selected elements used for what purpose.
Adding the specific elements to the copper core usually deteriorates its thermal conductivity rapidly. When the elements are added to the copper core to prepare a copper-alloyed core so as to employ it to a center electrode or an outer electrode, the thermal conductivity of the two electrodes reduces, and thus making it impossible to control the development of the void and to prevent the growth of the void. In general, the copper-alloyed core deteriorates a preignition resistant property when it is used for the center electrode. The copper-alloyed core usually causes to readily oxidize the nickel clad in a high temperature environment so as to deteriorate a spark-erosion resistant property when used for the outer electrode.
Therefore, it is an object of the invention to provide a copper-alloyed core which is capable of holding fine grain size in high temperature so as to prevent voids readily developed in the grain boundary, and holding a good thermal conductivity and a good physical strength in high temperature. By employing the copper-alloyed core to the center and outer electrodes, the preignition resistant property of the spark plug is enhanced to contribute to its extended service life.